Currently, optical fibers used for optical fiber communication systems which are generally spreading adopt a structure in which an outer periphery of one core is surrounded by a clad, and information is transmitted when an optical signal propagates in this core. Further, as the optical fiber communication systems spread, the amount of information to be transmitted is dramatically increasing in recent years. Following an increase in the amount of information to be transmitted, the optical fiber communication systems use several tens or several hundreds of multiple optical fibers to perform long-distance optical communication of a large volume.
It is known that, to reduce the number of optical fibers in such an optical fiber communication system, a plurality of signals are transmitted by means of light propagating in respective cores using a multi-core fiber in which outer peripheries of a plurality of cores are surrounded by one clad.
Non-Patent Document 1 below describes such a multi-core fiber. However, as also pointed out in Non-Patent Document 1, in the multi-core fiber in some cases, crosstalk occurs between optical signals propagating through cores, and noise is superposed on the optical signals propagating through the cores. In order to reduce such crosstalk, Non-Patent Document 2 below describes a so-called trench type multi-core fiber. In the structure of the trench type multi-core fiber, cores are individually surrounded by a first clad having a refractive index equivalent to or slightly different from the refractive index of a clad, the first clad is surrounded by a second clad having a refractive index lower than the refractive indices of the first clad and the clad, and the second clads are individually surrounded by the clad. In the case where the core, the first clad, and the second clad are considered to be a core element and the core element is seen from the viewpoint of the refractive indices, the multi-core fiber is a trench type because the refractive index of the second clad is in a trench. According to such a trench type multi-core fiber, light propagating through the core is more properly confined in the cores. Thus, it is suppressed that light propagating through the cores leaks out of the core element, and the crosstalk between the cores can be reduced.
Moreover, Non-Patent Document 3 below describes that in the case of hexagonal close packing in which a single core is disposed in the center and six cores surround the single core, it is likely that the crosstalk characteristics of the core disposed in the center is degraded.